This invention relates to optical communications using separation techniques to control individual wavelengths of dense wavelength division multiplexed beams, and more particularly to systems and methods which use liquid crystal spatial light modulation to efficiently modify individual wavelength components that have arbitrary states of polarization.
Next generation optical networks will focus on economically exploiting the inherent bandwidth of optical fiber. A primary technique now used for that purpose is dense wavelength division multiplexing (DWDM), which propagates a number of wavelength signals or channels each separated in accordance with standardized protocols to provide 25, 50, 100 GHz, etc., channel separation at specified data rates, in consequence of which the bandwidth of optical fiber is more effectively utilized. Optical routing, agile wavelength provisioning, and precise wavelength management are key characteristics needed for these next generation communication systems. These systems must provide the end user with the ability to perform attenuation, blocking and switching in a channel independent fashion.
Optical communication systems, however, impose a number of particular and demanding requirements on units that are to modify, block, split, equalize or add individual wavelengths. These requirements involve among other things the need for low chromatic dispersion, low polarization mode dispersion, low polarization dependent frequency and loss, and other factors which constitute, at various levels, unacceptable aberrations in optical signals. Such factors are becoming increasingly stringent as data rates are increased and wavelength separations are reduced. For example, even at 50 GHz separation, prior art units have difficulty in meeting existing performance needs on a per wavelength basis. Optical systems with these dense channel separations are even less tolerant of optical signal aberrations which compromise insertion loss and channel isolation. Additional performance factors, such as linear attenuation within a wide dynamic range and high extinction ratio, can also be of crucial importance, depending on the system application.
Various techniques are known for beam control along a pixel array, but the leading approach in the field of optical communications is to utilize liquid crystal spatial light modulators (LC-SLMs). LC-SLMs consist of an array of individual pixels or cells that can be individually controlled electrically and suitably miniaturized. Extensive development and production work has been done on these devices for large scale projection and flat panel television systems. The properties needed for optical communication cells and the special needs of data transmission, however, impose different demands. For optical communications, the cells can be used in analog fashion to function as attenuators, limiters, or equalizers, or they can also or separately be used in a digital fashion, in which case they function to control optical beams between essentially full transmission and full extinction ( greater than 40 dB) states. In display applications, the modulation of the optical properties of the cells can accommodate a degree of unwanted variations, because a display will still appear consistent or flicker-free to the viewer. Fiber optic applications, however, require a much more precise and uniform optical response and freedom from subtle aberrations, and therefore demand new and unique designs to meet optical systems requirements.
Operative cells in LC-SLM arrays can be disposed within existing optical modules, provided that they meet specific performance specifications, because they are sized and spaced for accepting spatially separated wavelength signals and have low power demands. Therefore they can conveniently be used in systems such as routers, multiplexers, demultiplexers and dynamic equalizers for multiple channels in optical networks. Systems with LC-SLM arrays can in turn be incorporated into broader system designs, so as to provide new system and method capabilities. For example, a new multichannel optical filter system is now known which disposes a complementary combination of Fourier optics and diffraction gratings in a very compact configuration employing multiple three dimensional folding at small angles of two dimensional beam patterns. The system generates, from an input wavelength multiplexed signal, a plurality of parallel, spatially and spectrally distributed but closely adjacent beams. Individual wavelength components can be modified statically, dynamically, or in a predetermined and preset fashion, and then can be recombined by three dimensional folding to provide a reconfigured WDM output. The introduction of a dynamic multi-cell control array to modulate the individual demultiplexed beams enables generation of a wide range of possibilities for new optical communications networks.
Liquid crystals for switch and/or attenuator applications have been described generically in numerous papers and patents. For example, Ranalli et al. (U.S. Pat. No. 6,285,500) describe a wavelength selective switch utilizing diffraction gratings, a twisted nematic LC cell, an LC-SLM linear array, and a complex four-port configuration with polarization independence achieved by splitting the s and p polarizations at the input into separated paths which propagate in parallel through the filter, to be recombined at the outputs by polarization beam splitters. Liquid crystal SLM arrays have been used in numerous designs of all-optical crossconnects. The techniques and processes to fabricate LC-SLM components are clearly quite advanced and are based on years of design and manufacturing development derived from flat panel displays. Patel (U.S. Pat. No. 6,252,644) describes a wedge shaped liquid crystal cell which provides high extinction by introducing a gradient in thickness of the twisted nematic liquid crystal cell in the lateral direction. The application of this type of cell to a WDM switch is described, for example, in the article xe2x80x9cLiquid crystal and grating based multiple wavelength cross connect switch,xe2x80x9d IEEE Photonics Technology Letters, vol. 7, no. 5, May 1995, pp. 514-516.
Wu et al. (U.S. Pat. Nos. 6,175,432 and 6,134,358) describe a multi-wavelength crossconnect utilizing polarization beam splitters, birefringent crystal filter elements and liquid crystal polarization rotators. Liu et al. (U.S. Pat. No. 6,208,442) describe a programmable optical multiplexer based on similar concepts. Wong et al. (U.S. Pat. No. 6,201,593) disclose a multiple wavelength, twisted nematic (TN) liquid crystal switch with an extinction ratio in excess of 25 dB, using a twisted nematic cell with specially angled LC alignment layers.
Pan (U.S. Pat. No. 6,181,846) describes a fiberoptic liquid crystal on-off switch and variable attenuator which operate in reflection mode. This device has a single input fiber and a single output fiber, and selectively directs all or a part of input beams into the output port.
The liquid crystal devices described in the prior art are highly specialized devices tailored to particular applications. Apart from these applications, they do not address the general needs of optical communications networks, or confront the specific problems faced where LC-SLMs are to be used with diffractive Fourier optical systems. The potential that these systems have for very flat passbands, sharp spectral roll-off, low polarization dependence, low adjacent channel crosstalk and low chromatic dispersion should not be diminished to any meaningful extent by the modulation techniques that are used. However, diffraction gratings do not distribute wavelength components uniformly in space, only inputs of arbitrary states of polarization are accepted, and the systems are so physically compact and beam paths so dense that minute variations in alignment or angle can alter operating performance in a negative manner. Furthermore, the liquid crystal cells themselves have unique characteristics which must be taken into account. They are both wavelength and temperature dependent, and their characteristics can also vary with age. Furthermore, their response deteriorates if the incident light intensity is in excess of a given threshold.
Important operative factors that apply to optical communication systems that employ spectrographic filtering arise in meeting passband requirements imposed by the very high and different data rates with which each wavelength may be modulated. The beams which are diffractively dispersed from a DWDM input not only have a Gaussian power intensity distribution about the center (e.g. laser) wavelength but are more widely distributed because of the sideband components. These sideband components must be preserved so that data is not compromised, but at the same time adequate stopband widths must be maintained. These passband and stopband requirements, in addition to the requirements for high efficiency response and linear modulation or full extinction at each wavelength, are not encountered in display and other types of systems that use liquid crystal elements.
Any optical system has what may be called a tolerance budget, representing the cumulative total of the critical alignments that must be observed for the system to function as designed. While many configurations may theoretically be feasible, excessive criticality in components, alignment, or assembly procedures can render a system impractical for production or use.
Systems and methods in accordance with the invention employ the phase retardation properties of voltage controllable liquid crystal cells in conjunction with polarization diversity optics proximate to the cells to effect controlled attenuation or extinction of individual beams of different wavelengths and initially arbitrary states of polarization. DWDM input beams to be separated into dispersed wavelength components by a diffractive Fourier optics system may be split into separate polarization components before or after wavelength dispersion. Whichever approach is used, the phase retardation introduced by the liquid crystal transforms the polarization of the wavelength component, such that one or more polarization dependent devices are used to reject a predetermined part of the signal at that wavelength. The optical axis or axes of the polarization management optics are in chosen relation to the alignment axis of the liquid crystal and, in some instances, to the polarization direction of the wavelength components. Separate polarization components for each wavelength may be superimposed, to be coincident on the liquid crystal surface, or they may be separate, but in any event the beam spots are distributed over a sufficiently large area to keep the local light intensity below a deleterious threshold throughout the area. The liquid crystal cells may be reflective or transmissive, and zero twist nematic or twisted nematic, although the zero twist nematic, reflective type is generally used for a compact diffractive Fourier optics system in which a linear attenuation range is a primary performance characteristic.
Beam polarization separation and recombination can be effected before and after diffractive dispersion recombination in an optical system where signals at denser channel spacings (e.g. 25 GHz) are being modulated or blocked. The polarization dependent components can be polarized sheets or plates, or polarization beam displacers, in reflected or transmitted beam paths.
In a more specific example in accordance with the invention, polarization components are split prior to initial wavelength separation of the beam, as by a Wollaston prism beamsplitter system which diverges the polarization components of the DWDM beam by a small separation angle. Both components are then propagated in close adjacency through multiple folds in the diffractive Fourier optics system and the beams are directed such that dispersed individual wavelength components converge as separate polarization components toward a liquid crystal array at a focal plane. The wavelength components are asymmetrical, being narrow and separate but closely adjacent in the sagittal direction, but of more than an order of magnitude longer in the transverse direction, with the polarization components forming separated beam waists of like polarization direction at the liquid crystal. The liquid crystal cells are zero twist nematic cells which have optical axes in a selected relation to the polarization direction of the beam components, such that they phase retard the incident beams to an extent determined by individual control signals at each cell. The reflected beams diverge at a low separation angle with, in general, elliptical polarization at some azimuth angle, which determines the percentage of signal to be rejected by adjacent polarization dependent elements, the rejection being affected by absorption or diversion. The two polarization component beams thereafter return reciprocally through the prior three dimensional folding paths for the polarization and wavelength components to be recombined, providing the modified DWDM signals.
Advantageously, using an SLM array of elongated LC cells, asymmetric, diffraction limited beam spots are formed having transverse dimensions in the range of 200-250 microns with the 1/e2 optical intensity of the carrier wavelength having a sagittal distribution in the range of 8-11 microns, and extending further sagittally because of the sideband components. The optical paths into the polarization management system are disposed such that the separate polarization components converge at slightly different angles to meet on the LC-SLM surface, and then reflect, switching paths as they return. Beam components that are rejected may be blocked by a polarizing plate or sheet of proper alignment, or alternatively be diverted by one or more polarization beam displacers (PBD). to an energy diffusing surface or to a separate return path.
Each pixel surface area is defined by its LC cell boundary, which is bordered by an interpixel gap that serves as a barrier to the adjacent pixel area. By employing a sagittal pixel width that is sufficient to accommodate the broadened sagittal width of the incident wavelength beam that has been modulated at a high data rate, the full optical intensity spectrum at that wavelength is modulated and reflected with high efficiency. By maintaining a chosen relationship between the pixel width and interpixel gap width, both the passband width and the stopband width characteristics needed for high performance are attained.
An alternative arrangement of polarization management and beam power control using liquid crystal cells in an SLM array may be advantageous at wider channel spacings. In this exemplification, the beam shapes are symmetrical or moderately asymmetrical, and the beam polarization components are separated after dispersion of the wavelength components at the first of a pair of polarization beam displacers. The optical axes of the PBDs are angled diagonally relative to each other and the alignment axes of the liquid crystal cells. The control voltages applied to the liquid crystals again determine the ellipticity and azimuth of the reflected beam components, so that the PBDs separate the signal components from lossy components that are to be rejected. Rejected components are diverted from the useful field of view, while the individual wavelength signals return through the beam refolding system to recombine as the modified DWDM signal.
Systems in accordance with the invention, which employ a shared input/output collimator and multiple forward and reverse refolding of beams along substantially equal focal length paths, are advantageous in that they are tolerant of small collimator misalignments. Coupling efficiency is maintained at a high level because pointing errors of the input beam are identically tracked by the output beam.
The polarization management optics can be configured in relation to the input/output optics of the diffractive Fourier optics system to employ the tight three-dimensional refolding capability of the system for greater application diversity. Thus a double collimator device, with collimators at closely spaced elevations, may be used together with polarization diversity optics which utilize a series of polarization beam displacers to reflect beams at either of two elevations, with an elevation difference corresponding to the input collimators. This system may be used, with individual cell commands, to provide interleaver, block switching, add/drop and other functions.
The responses of nematic liquid crystals are linearized in response to different operating conditions by using look-up tables or other reference data to correct for wavelength, temperature and other disparities. In the present system this may be further supplemented by employing a test cell in the array, and testing the capacitance of that cell, which provides data needed not only for temperature, but for aging as well as other parameters.
The stringent performance demands of modern optical systems are also met by use of specific designs and adjustments that minimize aberrations. PDL is minimized, for example, by the use of different but symmetrical polarization component beam paths which are superposed at the liquid crystal surface. High extinction values are attained by precisely (to less than 0.1xc2x0) aligning the optical axis of a polarized plate relative to the optical axis of a quarter waveplate interposed before the LC, which also ensures full transmission in the power-off condition. Chromatic dispersion is reduced by positioning the LC in the Fourier plane of the diffraction grating so that the forward and backward propagation are nominally identical in amplitude and phase profile for each individual wavelength signal. A Wollaston prism beam splitter is of benefit in assuring suitable separation and alignment of the polarization components. Multipath interference, group delay ripple and other performance factors affected by back reflections are minimized not only by the use of antireflection coatings, but also by tilting of entrance and exit surfaces on components to reject spurious reflections arising from surfaces in proximity to the liquid crystal modulator plane.
In another example of a multi-beam modulating system for providing gain or channel equalization of a DWDM signal, the liquid crystal array is subdivided along its axis into a multiple pixel per channel format. For example, the pixel pitch may be less than 2 microns, with many (e.g. 4096) pixels for an 80 channel wavelength dispersed DWDM beam. Zero twist nematic LCs forming a pixel cell array along a sagittal plane have their optical alignment axes oriented parallel to the sagittal direction. The sagittally distributed beam spots representing different wavelengths are incident across the entire array. The beam spot widths are substantially greater than the individual pixel widths and inter-pixel spacings, with 1/e2 optical impulse values within each beam spot thus being distributed over a number of pixels. The pixels are individually modulated, and the LC-SLM provides a smoothly distributed response that can be used in gain equalization applications and advantageously to tailor amplitude variations within channel passbands themselves.
Polarization diversity optics operating to provide beam control at different elevations can be used with compatible input/output structures to enhance the versatility of diffractive Fourier optics systems using three dimensional refolding. For example, different input and/or output DWDM beams that are spaced apart by millimeter-sized elevation differences can be communicated into and out of the diffractive Fourier optics system which separates and recombines the wavelength components in reciprocal refolding sequences. In the process, the individual wavelength signals may be separated by polarization diversity optics into separate polarization components which are incident at different elevations on an LC-SLM cell operated, for example, in fill extinction or full transmission mode. Dependent on the transformation used at an individual cell, the return output beams are at one of two elevation levels corresponding to the input/output differential. This enables the system to provide, in response to an input DWDM beam, separate express/drop outputs, and interleaver capability, or band splitting operation. This approach also is advantageously used with beam polarization splitters disposed between the input/output elements and the diffractive Fourier optics system, and superimposing convergent polarization components on each cell of a reflective zero twist nematic LC-SLM. In this example, the polarization diversity optics directs both polarization components back to the diffractive Fourier optics system at differential elevations such that, dependent on the transformation used at the LC cell, the wavelength signal is recombined with like-transformed other wavelengths at the appropriate signal path.